1) Field of the Invention
The invention relates to a process for removing silicones present on fibers, yarns or textile sheet materials, especially for the recovery and recycling of fibrous material from airbag production.
2) Description of Prior Art
To a higher and higher extent, silicones are employed for the surface treatment of fibers, yarns or textile sheet materials. In particular, silicones are used for the hydrophobization of textiles. The fabric treated with silicones frequently obtains a soft and smooth touch. When yarns are treated with silicones, especially silicone oils, the fiber/metal friction is reduced.
The liquid to paste-like chain-shaped dialkylpolysiloxanes, especially dimethylpolysiloxanes, the methylhydrogenpolysiloxanes that are fixable on the fiber and silicone resins containing tri- or tetraflnctional silicon moieties and being condensable onto the fiber are used as the silicones.
The fact that organosilicones are difficult to remove from the fiber so that the silicone impregnation is fast to washing and cleaning is of particular advantage. However, this desired property can be very disturbing when it becomes necessary in practice to strip the silicone impregnation from defective lots: In “Chemie und Technologie der Silikone” by Walter Noll, Verlag Chemie GmbH, 2nd Edition, 1968, p. 512, it is stated that the silicone film is to be decomposed in a hot and acidic environment, wherein a combination with solvent-containing detergents can be contemplated. It is not surprising that such an aggressive procedure can often result in damage to the fibers.
EP 0950684 B1 describes a process for the reprocessing of polyamide material containing silicone resin using a 5 to 50% by weight alkali hydroxide solution. From DE 35 15 077 C1, it is known to remove a silicone impregnation from textile materials by treating the textile materials with aqueous formulations containing detergent substances, oxalic acid and sodium carbonate. Although this procedure is slightly more gentle, it does not allow the silicones to be removed completely from the textile surfaces and fails especially when the silicones are cross-linked into products having a high molecular weight.
From this document, there is also already known a process for removing silicones present on fibers, yarns or textile sheet materials, characterized in that aqueous preparations containing surface-active equilibration catalysts for organosiloxanes that are capable of cleaving and reforming siloxane bonds in amounts of from 0.2 to 5% by weight, based on the aqueous preparation, are allowed to act on the substrate, optionally at elevated temperatures. With this process, it is possible to remove silicone impregnations from the surface of fibers, yarns or textile sheet materials under as mild as possible conditions. The thus treated textile materials can then again have a wetting behavior like they had before the impregnation in order to optionally subject them to further treatments, for example, to provide a new color or shade.
However, the process described in DE 35 15 077 C1 reaches its limits where thick layers of highly cross-linked silicone of high density must be removed. Especially in the production/ready-making of airbags for the vehicle industry, textile materials coated in this way are used because their highly cross-linked and adhesive-free coating is required for retaining the explosion gases occurring when the airbag is triggered and for enabling inflation of the airbag. For the fabric coating, especially for the airbag finishing, liquid silicone rubbers which are two-component systems and consist of a methylhydrogensiloxane as a cross-linking agent and a silicone polymer containing vinyl groups that vulcanize under the catalysis of Pt(0) complexes by means of a hydrosilylation reaction are usually employed. The two components are mixed and applied to the fabric in a thin layer by a suitable coating process, for example, with a knife-over-roll. The vulcanization of the silicone rubber is effected at temperatures of from 130 to 170° C., and annealing is often performed afterwards at 200° C.
For very smooth and dense fabrics of polyamide or other synthetic fibers, the adhesion of the coating composition is not optimal, so that priming with suitable organosilanes for improving the adhesion is often necessary.
The large supply with airbags, which has become ubiquitous in the meantimne, and the growing production of waste from airbag production/fabric scraps from airbag ready-making as well as used airbags from the junking of motor vehicles result in a need for a process for the processing and recycling of the raw materials employed, in this case the fibrous material and silicones employed. For example, in Great Britain alone, 4,000 tons of silicone-coated fabrics derived from airbags or airbag production are currently obtained per year, for which a useful recycling process is not available in the prior art.